The purpose of the proposed research is to examine the effect of fitness differences which arise as a result of social interactions among relatives. Evolutionary changes occurring under kin selection will be studied using mathematical population genetics models. In addition to the analysis of the dynamics of genotype frequency evolution, two questions are of particular significance to the theory of kin selection: 1) What is the measure of genetic relatedness known as Hamiltion's r? 2) To what extent are the predictions of the heuristic theory of kin selection correct in the case of one locus selection? Mathematical models in the following three broad areas will be studied. 1) Evolution of the sex ratio: Analysis will focus on the extent to which genetic relatedness among siblings controls the value to which the sex ratio will evolve under sibling (sister or brother) control. 2) Parent-offspring interactions: Models in which parents sacrifice some aspect of their fitness in order to improve the fitness of offspring will be studied. Assumptions as to whether the costs and benefits alter viability or fertility affect the form of the model. In addition, "manipulation" models in which the costs and benefits associated with altruism are borne or received by individuals other than those controlling the social interaction will be studied. 3) Mating systems and kin selection: The genetic relatedness among relatives may be increased or decreased compared to the random mating case under various sytems of mating. Models involving multiple insemination, sib-mating, and positive and negative assortative mating will be studied.